I have a brand new DELL BLADE 1855 server which requires a L6-30P
plug.  This unit is a 208V device ( actually 200-240 V ).

There's an APC UPS with an L5-30P plug already installed in the rack,
which I am removing and no longer using.  This unit is a 110 V device.

I understand that these plugs are typically X,Y,Ground connections
with 110 in X, 110 in Y plus the ground.

I am asusming that the L5-30P only use the X or Y and the L6-30P uses
both to make the 220 V it requires.

Will I be able to just un-plug the UPS and plug-in the DELL BLADE server ?

(note: the actual UPS is redundant because this rack is in a major
server farm which already has full power failure protection so I dont
really need it any more).

FYI, the UPS is the following model:

http://www.apc.com/resource/include/techspec_index.cfm?base_sku=SU2200RMXLNET

FYI, the blade is the following model:

http://www1.us.dell.com/content/products/productdetails.aspx/pedge_1855?c=us&cs=555&l=en&s=biz

douglas69-ga:

You are not correct in your assumption. The L5-30 is rated @ 125V.
This server will not operate at that voltage and could be damaged.
Save yourself some time and money - hire a licensed electrician.

You can't get 208V out of that UPS. nfla-ga is right, hire an
electrician or call APC and order the correct UPS for your needs.

While I agree with the previous comments regarding hiring an
electrician, I am not sure that they answered your question, which I
interpret to mean: Is there 208V present on the plug presented to the
APC, and the APC is just not using it?

What you  could do to answer this question is to check the voltage
across the various plug pins with a volt meter. Just make sure you set
it to AC, and to a high voltage range (1000V) to start. That will tell
you if there is in fact 208V accross the "X" and "Y" pins as you call
them. (Personally I doubt it.)

Even so, you may need more AMPS than the circuit breaker (or wiring) will provide.
An electriciian will help you there. 
As I understand things, if you screw up your own electrical and cause
a fire, your insurance company will be very reluctant to pay out.